package com.fenyin.samples.study.jdk.nio.channels.udp;

import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.channels.DatagramChannel;
import java.util.Date;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;

/**
 * Request time service, per RFC 868. RFC 868
 * (http://www.ietf.org/rfc/rfc0868.txt) is a very simple time protocol whereby
 * one system can request the current time from another system. Most Linux, BSD
 * and Solaris systems provide RFC 868 time service on port 37. This simple
 * program will inter-operate with those. The National Institute of Standards
 * and Technology (NIST) operates a public time server at time.nist.gov.
 * 
 * The RFC 868 protocol specifies a 32 bit unsigned value be sent, representing
 * the number of seconds since Jan 1, 1900. The Java epoch begins on Jan 1, 1970
 * (same as unix) so an adjustment is made by adding or subtracting
 * 2,208,988,800 as appropriate. To avoid shifting and masking, a four-byte
 * slice of an eight-byte buffer is used to send/recieve. But getLong( ) is done
 * on the full eight bytes to get a long value.
 * 
 * When run, this program will issue time requests to each hostname given on the
 * command line, then enter a loop to receive packets. Note that some requests
 * or replies may be lost, which means this code could block forever.
 * 
 * @author Ron Hitchens (ron@ronsoft.com) 112
 */
public class TimeClient {
	
	private static final int DEFAULT_TIME_PORT = 37;
	private static final long DIFF_1900 = 2208988800L;
	protected int port = DEFAULT_TIME_PORT;
	@SuppressWarnings("rawtypes")
	protected List remoteHosts;
	protected DatagramChannel channel;

	public TimeClient(String[] argv) throws Exception {
		if (argv.length == 0) {
			throw new Exception("Usage: [ -p port ] host ...");
		}
		parseArgs(argv);
		this.channel = DatagramChannel.open();
	}

	protected InetSocketAddress receivePacket(DatagramChannel channel,
			ByteBuffer buffer) throws Exception {
		buffer.clear();
		// Receive an unsigned 32-bit, big-endian value
		//接受数据
		return ((InetSocketAddress) channel.receive(buffer));
	}

	// Send time requests to all the supplied hosts
	@SuppressWarnings("rawtypes")
	protected void sendRequests() throws Exception {
		ByteBuffer buffer = ByteBuffer.allocate(1);
		Iterator it = remoteHosts.iterator();
		while (it.hasNext()) {
			InetSocketAddress sa = (InetSocketAddress) it.next();
			System.out.println("Requesting time from " + sa.getHostName() + ":"
					+ sa.getPort());
			// Make it empty (see RFC868)
			buffer.clear().flip();
			// Fire and forget
			//发送数据
			channel.send(buffer, sa);

		}
	}

	// Receive any replies that arrive
	public void getReplies() throws Exception {
		// Allocate a buffer to hold a long value
		ByteBuffer longBuffer = ByteBuffer.allocate(8);
		// Assure big-endian (network) byte order
		longBuffer.order(ByteOrder.BIG_ENDIAN);
		// Zero the whole buffer to be sure
		longBuffer.putLong(0, 0);
		// Position to first byte of the low-order 32 bits
		longBuffer.position(4);
		// Slice the buffer; gives view of the low-order 32 bits
		ByteBuffer buffer = longBuffer.slice();
		int expect = remoteHosts.size();
		int replies = 0;
		System.out.println("");
		System.out.println("Waiting for replies...");
		while (true) {
			InetSocketAddress sa;
			sa = receivePacket(channel, buffer);
			buffer.flip();
			replies++;
			printTime(longBuffer.getLong(0), sa);
			if (replies == expect) {
				System.out.println("All packets answered");
				break;
			}
			// Some replies haven't shown up yet
			System.out.println("Received " + replies + " of " + expect + " replies");
		}
	}

	// Print info about a received time reply
	protected void printTime(long remote1900, InetSocketAddress sa) {
		// local time as seconds since Jan 1, 1970 114

		long local = System.currentTimeMillis() / 1000;
		// remote time as seconds since Jan 1, 1970
		long remote = remote1900 - DIFF_1900;
		Date remoteDate = new Date(remote * 1000);
		Date localDate = new Date(local * 1000);
		long skew = remote - local;
		System.out.println("Reply from " + sa.getHostName() + ":"
				+ sa.getPort());
		System.out.println(" there: " + remoteDate);
		System.out.println(" here: " + localDate);
		System.out.print(" skew: ");
		if (skew == 0) {
			System.out.println("none");
		} else if (skew > 0) {
			System.out.println(skew + " seconds ahead");
		} else {
			System.out.println((-skew) + " seconds behind");
		}
	}

	@SuppressWarnings({ "rawtypes", "unchecked" })
	protected void parseArgs(String[] argv) {
		remoteHosts = new LinkedList();
		for (int i = 0; i < argv.length; i++) {
			String arg = argv[i];
			// Send client requests to the given port
			if (arg.equals("-p")) {
				i++;
				this.port = Integer.parseInt(argv[i]);
				continue;
			}
			// Create an address object for the hostname
			InetSocketAddress sa = new InetSocketAddress(arg, port);
			// Validate that it has an address
			if (sa.getAddress() == null) {
				System.out.println("Cannot resolve address: " + arg);
				continue;
			}

			remoteHosts.add(sa);
		}
	}

	// --------------------------------------------------------------
	public static void main(String[] argv) throws Exception {
		TimeClient client = new TimeClient(argv);
		client.sendRequests();
		client.getReplies();
	}
}
